KeyedCollection<TKey,TItem>.GetKeyForItem(TItem) Method
Definition
Important
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When implemented in a derived class, extracts the key from the specified element.
protected:
abstract TKey GetKeyForItem(TItem item);protected abstract TKey GetKeyForItem (TItem item);abstract member GetKeyForItem : 'Item -> 'KeyProtected MustOverride Function GetKeyForItem (item As TItem) As TKeyParameters
- item
- TItem
The element from which to extract the key.
Returns
TKey
The key for the specified element.
Examples
This code example shows the minimum code necessary to derive a collection class from KeyedCollection<TKey,TItem>: overriding the GetKeyForItem method and providing a public constructor that delegates to a base class constructor. The code example also demonstrates many of the properties and methods inherited from KeyedCollection<TKey,TItem> and Collection<T> classes.
The SimpleOrder class is a very simple requisition list that contains OrderItem objects, each of which represents a line item in the order. The key of OrderItem is immutable, an important consideration for classes that derive from KeyedCollection<TKey,TItem>. For a code example that uses mutable keys, see ChangeItemKey.
using namespace System;
using namespace System::Collections::Generic;
using namespace System::Collections::ObjectModel;
// This class represents a simple line item in an order. All the
// values are immutable except quantity.
//
public ref class OrderItem
{
private:
int _quantity;
public:
initonly int PartNumber;
initonly String^ Description;
initonly double UnitPrice;
OrderItem(int partNumber, String^ description,
int quantity, double unitPrice)
{
this->PartNumber = partNumber;
this->Description = description;
this->Quantity = quantity;
this->UnitPrice = unitPrice;
}
property int Quantity
{
int get() { return _quantity; }
void set(int value)
{
if (value < 0)
throw gcnew ArgumentException("Quantity cannot be negative.");
_quantity = value;
}
}
virtual String^ ToString() override
{
return String::Format(
"{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}",
PartNumber, _quantity, Description, UnitPrice,
UnitPrice * _quantity);
}
};
// This class represents a very simple keyed list of OrderItems,
// inheriting most of its behavior from the KeyedCollection and
// Collection classes. The immediate base class is the constructed
// type KeyedCollection<int, OrderItem>. When you inherit
// from KeyedCollection, the second generic type argument is the
// type that you want to store in the collection -- in this case
// OrderItem. The first type argument is the type that you want
// to use as a key. Its values must be calculated from OrderItem;
// in this case it is the int field PartNumber, so SimpleOrder
// inherits KeyedCollection<int, OrderItem>.
//
public ref class SimpleOrder : KeyedCollection<int, OrderItem^>
{
// The parameterless constructor of the base class creates a
// KeyedCollection with an internal dictionary. For this code
// example, no other constructors are exposed.
//
public:
SimpleOrder() {}
// This is the only method that absolutely must be overridden,
// because without it the KeyedCollection cannot extract the
// keys from the items. The input parameter type is the
// second generic type argument, in this case OrderItem, and
// the return value type is the first generic type argument,
// in this case int.
//
protected:
virtual int GetKeyForItem(OrderItem^ item) override
{
// In this example, the key is the part number.
return item->PartNumber;
}
};
public ref class Demo
{
public:
static void Main()
{
SimpleOrder^ weekly = gcnew SimpleOrder();
// The Add method, inherited from Collection, takes OrderItem.
//
weekly->Add(gcnew OrderItem(110072674, "Widget", 400, 45.17));
weekly->Add(gcnew OrderItem(110072675, "Sprocket", 27, 5.3));
weekly->Add(gcnew OrderItem(101030411, "Motor", 10, 237.5));
weekly->Add(gcnew OrderItem(110072684, "Gear", 175, 5.17));
Display(weekly);
// The Contains method of KeyedCollection takes the key,
// type, in this case int.
//
Console::WriteLine("\nContains(101030411): {0}",
weekly->Contains(101030411));
// The default Item property of KeyedCollection takes a key.
//
Console::WriteLine("\nweekly(101030411)->Description: {0}",
weekly[101030411]->Description);
// The Remove method of KeyedCollection takes a key.
//
Console::WriteLine("\nRemove(101030411)");
weekly->Remove(101030411);
Display(weekly);
// The Insert method, inherited from Collection, takes an
// index and an OrderItem.
//
Console::WriteLine("\nInsert(2, New OrderItem(...))");
weekly->Insert(2, gcnew OrderItem(111033401, "Nut", 10, .5));
Display(weekly);
// The default Item property is overloaded. One overload comes
// from KeyedCollection<int, OrderItem>; that overload
// is read-only, and takes Integer because it retrieves by key.
// The other overload comes from Collection<OrderItem>, the
// base class of KeyedCollection<int, OrderItem>; it
// retrieves by index, so it also takes an Integer. The compiler
// uses the most-derived overload, from KeyedCollection, so the
// only way to access SimpleOrder by index is to cast it to
// Collection<OrderItem>. Otherwise the index is interpreted
// as a key, and KeyNotFoundException is thrown.
//
Collection<OrderItem^>^ coweekly = weekly;
Console::WriteLine("\ncoweekly[2].Description: {0}",
coweekly[2]->Description);
Console::WriteLine("\ncoweekly[2] = gcnew OrderItem(...)");
coweekly[2] = gcnew OrderItem(127700026, "Crank", 27, 5.98);
OrderItem^ temp = coweekly[2];
// The IndexOf method inherited from Collection<OrderItem>
// takes an OrderItem instead of a key
//
Console::WriteLine("\nIndexOf(temp): {0}", weekly->IndexOf(temp));
// The inherited Remove method also takes an OrderItem.
//
Console::WriteLine("\nRemove(temp)");
weekly->Remove(temp);
Display(weekly);
Console::WriteLine("\nRemoveAt(0)");
weekly->RemoveAt(0);
Display(weekly);
}
private:
static void Display(SimpleOrder^ order)
{
Console::WriteLine();
for each( OrderItem^ item in order )
{
Console::WriteLine(item);
}
}
};
void main()
{
Demo::Main();
}
/* This code example produces the following output:
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
101030411 10 Motor at 237.50 = 2,375.00
110072684 175 Gear at 5.17 = 904.75
Contains(101030411): True
weekly(101030411)->Description: Motor
Remove(101030411)
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
Insert(2, New OrderItem(...))
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
111033401 10 Nut at .50 = 5.00
110072684 175 Gear at 5.17 = 904.75
coweekly(2)->Description: Nut
coweekly[2] = gcnew OrderItem(...)
IndexOf(temp): 2
Remove(temp)
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
RemoveAt(0)
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
*/
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
// This class represents a very simple keyed list of OrderItems,
// inheriting most of its behavior from the KeyedCollection and
// Collection classes. The immediate base class is the constructed
// type KeyedCollection<int, OrderItem>. When you inherit
// from KeyedCollection, the second generic type argument is the
// type that you want to store in the collection -- in this case
// OrderItem. The first type argument is the type that you want
// to use as a key. Its values must be calculated from OrderItem;
// in this case it is the int field PartNumber, so SimpleOrder
// inherits KeyedCollection<int, OrderItem>.
//
public class SimpleOrder : KeyedCollection<int, OrderItem>
{
// This is the only method that absolutely must be overridden,
// because without it the KeyedCollection cannot extract the
// keys from the items. The input parameter type is the
// second generic type argument, in this case OrderItem, and
// the return value type is the first generic type argument,
// in this case int.
//
protected override int GetKeyForItem(OrderItem item)
{
// In this example, the key is the part number.
return item.PartNumber;
}
}
public class Demo
{
public static void Main()
{
SimpleOrder weekly = new SimpleOrder();
// The Add method, inherited from Collection, takes OrderItem.
//
weekly.Add(new OrderItem(110072674, "Widget", 400, 45.17));
weekly.Add(new OrderItem(110072675, "Sprocket", 27, 5.3));
weekly.Add(new OrderItem(101030411, "Motor", 10, 237.5));
weekly.Add(new OrderItem(110072684, "Gear", 175, 5.17));
Display(weekly);
// The Contains method of KeyedCollection takes the key,
// type, in this case int.
//
Console.WriteLine("\nContains(101030411): {0}",
weekly.Contains(101030411));
// The default Item property of KeyedCollection takes a key.
//
Console.WriteLine("\nweekly[101030411].Description: {0}",
weekly[101030411].Description);
// The Remove method of KeyedCollection takes a key.
//
Console.WriteLine("\nRemove(101030411)");
weekly.Remove(101030411);
Display(weekly);
// The Insert method, inherited from Collection, takes an
// index and an OrderItem.
//
Console.WriteLine("\nInsert(2, New OrderItem(...))");
weekly.Insert(2, new OrderItem(111033401, "Nut", 10, .5));
Display(weekly);
// The default Item property is overloaded. One overload comes
// from KeyedCollection<int, OrderItem>; that overload
// is read-only, and takes Integer because it retrieves by key.
// The other overload comes from Collection<OrderItem>, the
// base class of KeyedCollection<int, OrderItem>; it
// retrieves by index, so it also takes an Integer. The compiler
// uses the most-derived overload, from KeyedCollection, so the
// only way to access SimpleOrder by index is to cast it to
// Collection<OrderItem>. Otherwise the index is interpreted
// as a key, and KeyNotFoundException is thrown.
//
Collection<OrderItem> coweekly = weekly;
Console.WriteLine("\ncoweekly[2].Description: {0}",
coweekly[2].Description);
Console.WriteLine("\ncoweekly[2] = new OrderItem(...)");
coweekly[2] = new OrderItem(127700026, "Crank", 27, 5.98);
OrderItem temp = coweekly[2];
// The IndexOf method inherited from Collection<OrderItem>
// takes an OrderItem instead of a key
//
Console.WriteLine("\nIndexOf(temp): {0}", weekly.IndexOf(temp));
// The inherited Remove method also takes an OrderItem.
//
Console.WriteLine("\nRemove(temp)");
weekly.Remove(temp);
Display(weekly);
Console.WriteLine("\nRemoveAt(0)");
weekly.RemoveAt(0);
Display(weekly);
}
private static void Display(SimpleOrder order)
{
Console.WriteLine();
foreach( OrderItem item in order )
{
Console.WriteLine(item);
}
}
}
// This class represents a simple line item in an order. All the
// values are immutable except quantity.
//
public class OrderItem
{
public readonly int PartNumber;
public readonly string Description;
public readonly double UnitPrice;
private int _quantity = 0;
public OrderItem(int partNumber, string description,
int quantity, double unitPrice)
{
this.PartNumber = partNumber;
this.Description = description;
this.Quantity = quantity;
this.UnitPrice = unitPrice;
}
public int Quantity
{
get { return _quantity; }
set
{
if (value<0)
throw new ArgumentException("Quantity cannot be negative.");
_quantity = value;
}
}
public override string ToString()
{
return String.Format(
"{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}",
PartNumber, _quantity, Description, UnitPrice,
UnitPrice * _quantity);
}
}
/* This code example produces the following output:
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
101030411 10 Motor at 237.50 = 2,375.00
110072684 175 Gear at 5.17 = 904.75
Contains(101030411): True
weekly[101030411].Description: Motor
Remove(101030411)
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
Insert(2, New OrderItem(...))
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
111033401 10 Nut at .50 = 5.00
110072684 175 Gear at 5.17 = 904.75
coweekly[2].Description: Nut
coweekly[2] = new OrderItem(...)
IndexOf(temp): 2
Remove(temp)
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
RemoveAt(0)
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
*/
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
' This class represents a very simple keyed list of OrderItems,
' inheriting most of its behavior from the KeyedCollection and
' Collection classes. The immediate base class is the constructed
' type KeyedCollection(Of Integer, OrderItem). When you inherit
' from KeyedCollection, the second generic type argument is the
' type that you want to store in the collection -- in this case
' OrderItem. The first generic argument is the type that you want
' to use as a key. Its values must be calculated from OrderItem;
' in this case it is the Integer field PartNumber, so SimpleOrder
' inherits KeyedCollection(Of Integer, OrderItem).
'
Public Class SimpleOrder
Inherits KeyedCollection(Of Integer, OrderItem)
' This is the only method that absolutely must be overridden,
' because without it the KeyedCollection cannot extract the
' keys from the items. The input parameter type is the
' second generic type argument, in this case OrderItem, and
' the return value type is the first generic type argument,
' in this case Integer.
'
Protected Overrides Function GetKeyForItem( _
ByVal item As OrderItem) As Integer
' In this example, the key is the part number.
Return item.PartNumber
End Function
End Class
Public Class Demo
Public Shared Sub Main()
Dim weekly As New SimpleOrder()
' The Add method, inherited from Collection, takes OrderItem.
'
weekly.Add(New OrderItem(110072674, "Widget", 400, 45.17))
weekly.Add(New OrderItem(110072675, "Sprocket", 27, 5.3))
weekly.Add(New OrderItem(101030411, "Motor", 10, 237.5))
weekly.Add(New OrderItem(110072684, "Gear", 175, 5.17))
Display(weekly)
' The Contains method of KeyedCollection takes TKey.
'
Console.WriteLine(vbLf & "Contains(101030411): {0}", _
weekly.Contains(101030411))
' The default Item property of KeyedCollection takes the key
' type, Integer.
'
Console.WriteLine(vbLf & "weekly(101030411).Description: {0}", _
weekly(101030411).Description)
' The Remove method of KeyedCollection takes a key.
'
Console.WriteLine(vbLf & "Remove(101030411)")
weekly.Remove(101030411)
Display(weekly)
' The Insert method, inherited from Collection, takes an
' index and an OrderItem.
'
Console.WriteLine(vbLf & "Insert(2, New OrderItem(...))")
weekly.Insert(2, New OrderItem(111033401, "Nut", 10, .5))
Display(weekly)
' The default Item property is overloaded. One overload comes
' from KeyedCollection(Of Integer, OrderItem); that overload
' is read-only, and takes Integer because it retrieves by key.
' The other overload comes from Collection(Of OrderItem), the
' base class of KeyedCollection(Of Integer, OrderItem); it
' retrieves by index, so it also takes an Integer. The compiler
' uses the most-derived overload, from KeyedCollection, so the
' only way to access SimpleOrder by index is to cast it to
' Collection(Of OrderItem). Otherwise the index is interpreted
' as a key, and KeyNotFoundException is thrown.
'
Dim coweekly As Collection(Of OrderItem) = weekly
Console.WriteLine(vbLf & "coweekly(2).Description: {0}", _
coweekly(2).Description)
Console.WriteLine(vbLf & "coweekly(2) = New OrderItem(...)")
coweekly(2) = New OrderItem(127700026, "Crank", 27, 5.98)
Dim temp As OrderItem = coweekly(2)
' The IndexOf method, inherited from Collection(Of OrderItem),
' takes an OrderItem instead of a key.
'
Console.WriteLine(vbLf & "IndexOf(temp): {0}", _
weekly.IndexOf(temp))
' The inherited Remove method also takes an OrderItem.
'
Console.WriteLine(vbLf & "Remove(temp)")
weekly.Remove(temp)
Display(weekly)
Console.WriteLine(vbLf & "RemoveAt(0)")
weekly.RemoveAt(0)
Display(weekly)
End Sub
Private Shared Sub Display(ByVal order As SimpleOrder)
Console.WriteLine()
For Each item As OrderItem In order
Console.WriteLine(item)
Next item
End Sub
End Class
' This class represents a simple line item in an order. All the
' values are immutable except quantity.
'
Public Class OrderItem
Public ReadOnly PartNumber As Integer
Public ReadOnly Description As String
Public ReadOnly UnitPrice As Double
Private _quantity As Integer = 0
Public Sub New(ByVal partNumber As Integer, _
ByVal description As String, _
ByVal quantity As Integer, _
ByVal unitPrice As Double)
Me.PartNumber = partNumber
Me.Description = description
Me.Quantity = quantity
Me.UnitPrice = unitPrice
End Sub
Public Property Quantity() As Integer
Get
Return _quantity
End Get
Set
If value < 0 Then
Throw New ArgumentException("Quantity cannot be negative.")
End If
_quantity = value
End Set
End Property
Public Overrides Function ToString() As String
Return String.Format( _
"{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}", _
PartNumber, _quantity, Description, UnitPrice, _
UnitPrice * _quantity)
End Function
End Class
' This code example produces the following output:
'
'110072674 400 Widget at 45.17 = 18,068.00
'110072675 27 Sprocket at 5.30 = 143.10
'101030411 10 Motor at 237.50 = 2,375.00
'110072684 175 Gear at 5.17 = 904.75
'
'Contains(101030411): True
'
'weekly(101030411).Description: Motor
'
'Remove(101030411)
'
'110072674 400 Widget at 45.17 = 18,068.00
'110072675 27 Sprocket at 5.30 = 143.10
'110072684 175 Gear at 5.17 = 904.75
'
'Insert(2, New OrderItem(...))
'
'110072674 400 Widget at 45.17 = 18,068.00
'110072675 27 Sprocket at 5.30 = 143.10
'111033401 10 Nut at .50 = 5.00
'110072684 175 Gear at 5.17 = 904.75
'
'coweekly(2).Description: Nut
'
'coweekly(2) = New OrderItem(...)
'
'IndexOf(temp): 2
'
'Remove(temp)
'
'110072674 400 Widget at 45.17 = 18,068.00
'110072675 27 Sprocket at 5.30 = 143.10
'110072684 175 Gear at 5.17 = 904.75
'
'RemoveAt(0)
'
'110072675 27 Sprocket at 5.30 = 143.10
'110072684 175 Gear at 5.17 = 904.75
Remarks
If the key in the lookup dictionary is different from the key that is embedded in item, you cannot access item by using the key returned by GetKeyForItem.
You can implement this method to return null for a collection that contains items without keys, in which case the items can be accessed only by their index. This method is an O(1) operation.
Notes for Implementers
You must override this method to provide the dictionary with a way to extract keys from items in the dictionary.
This method is called internally. It is not necessary for it to be public.
Applies to
See also
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